Why does a violin produce a louder, richer, more musical sound than a violin string stretched between two bricks? Because of sympathetic vibration and resonance.
Sound travels through the air in waves. Waves are made up of alternate pulses of pressure and release. As a string (or anything else) vibrates, it alternately compresses and stretches the molecules of air around it. The resultant sound-waves behave like any other wave forms. They are subject to laws of reflection, refraction, and decay. Could we see them as they reflect off suitably hard surfaces in a room, we would see patterns like those of ripples in a bath of water - criss-crossing, cancelling each other out or reinforcing each other, small waves riding on bigger waves underneath, and so on. The speed at which any sound wave travels (its velocity) is fixed by the density and molecular characteristics of the substance through which it passes.
These air waves eventually bombard our ear drums, causing them to vibrate in a pattern similar to that of the string. They also bombard anything else in their path. When they hit a concrete wall, for the most part they just bounce off. If they hit another stretched string identical to the first, its elasticity allows it to respond more sympathetically. The waves travel through the string. As they do so, the peaks and troughs of pressure encourage this second string to start vibrating in a pattern identical to that of the first. This is sympathetic vibration. If the sound of the first string is stopped, the second will continue to vibrate and make a sound on its own.
The same principle applies to any other object or substance with a suitably elastic molecular structure. The wooden body of a violin picks up vibrations from the strings directly above, and vibrates in sympathy. In so doing, the natural characteristics of the wood, its shape and size, all affect the quality of the eventual sound. In addition, the air cavity inside the wooden body sends the sound waves zapping round and bouncing off the inner walls. It thus sets up its own increasingly complex wave patterns.
So the sound we hear from the violin is not just that of the vibrating string. The body of the violin provides a chamber which effectively amplifies and enhances the original sound. This is known as resonance.
